1. Field of the Invention
This invention relates to steel cords usable as a reinforcing material for rubber articles such as pneumatic tires, conveyer belts and the like, and more particularly to a steel cord having a high tensile strength and excellent ductility for attaining reduction of weight and improvement of durability in a composite rubber body.
2. Description of the Related Art
In order to reduce the weight of the rubber article, it is advantageous to increase the tensile strength of the steel cord to reinforce the rubber article with less or thinner steel cords. On the other hand, in order to improve the durability of the rubber article, it is advantageous to improve the fatigue resistance of the steel cord. For this end, it is strongly demanded to improve the tensile strength and fatigue resistance of the steel cord. Further, in order to economically produce such steel cords, it is important to ensure the ductility of each steel filament wire constituting the steel cord to prevent the occurrence of wire breakage during twisting.
As a technique for improving the tensile strength of the steel cord, there are proposed a technique wherein a carbon content of a wire rod as a starting material for the steel cord is made higher than that of an ordinary wire rod, a technique wherein a wire drawing ratio of the wire rod is increased, and the like. However, the increase of the carbon content or the wire drawing ratio in the wire rod produces a new problem of degrading the fatigue resistance of the steel cord. For this end, in order to improve the fatigue resistance of the steel cord, it is attempted to render the composition of the wire rod as a starting material for the steel cord into a high alloy to form a fine pearlite structure, or to decrease the amount of non-metal inclusion in the wire rod, or the like.
However, expensive elements are used in alloying, so that the cost of the starting material for the steel cord undesirably rises. On the other hand, when the non-metal inclusion is decreased in the wire rod as far as possible, the production steps become complicated to increase the production cost. In any case, these techniques are disadvantageous in view of economical reasons. Even when such a wire rod is used to produce steel cords, the high tensile strength and good fatigue resistance are not necessarily satisfied at the same time. Especially, the degradation of the fatigue resistance is a serious problem in rubber articles such as tires and the like subjected to repetitive bending under loading because some of the steel filament wires constituting the steel cord are broken by the repetitive bending to bring about the occurrence of cord breakage and hence result in fatigue breakage of the composite rubber body or the tire.